I found the ruin but it was not clear if the original structure was a chamber. To me, the size, shape, entrance and the way that earth was piled on the side walls suggest that the ruin might have been a chamber.
About 200m N of the ruin there were two cairns marked on the map so I decided to look at them before heading back. The two cairns were actually part of a group of rock piles. I continued W from these cairns and came across more and more cairns. By the time I decided to leave I had found about 12 - 15 cairns in a 400m x 100m area situated in a saddle between a low hill to the SE and a high ridge/hill to the NW. The cairns showed a surprising variety in size and construction.
There were also a number of interesting stone walls in the area. The walls ran across the saddle and did not form any grids or enclosures.
There are three more cairns marked on the orienteering map (up the slope of the high ridge) so that I feel that there could be many more cairns in the area as it appears that only the most significant cairns were mapped.
Although the woods and bugs are thick, and it’s a hike to get there I think I'll go back to do some more exploring.